1. Field of the Invention
The present invention relates to a process for producing plastic foam made of a foaming resin composition containing a propylene-based unsaturated copolymer resin obtained by random copolymerization of propylene, ethylene, and a specific non-conjugated diene, and the foams therefrom.
2. Discussion of the Background
Polypropylene foam has been used in a wide variety fields because of its heat insulating properties, cushioning properties, lightness, etc.
Generally employed techniques for obtaining a foam of a high expansion-rate include a process utilizing crosslinking to improve foam stability at the time of foaming and a process in which a resin containing a large quantity of a physical foaming agent (pneumatogen) is made to foam before the pneumatogen is volatilized.
Olefin resins, inter alia, propylene resins are excellent in moldability, transparency, heat resistance, electrical insulating properties, water resistance, chemical resistance, mechanical strength, and the like and are therefore used in a many fields. However, satisfactory foams cannot be obtained from .alpha.-olefin resins, including propylene resins. While crosslinking is desired for producing foams from this kind of resins, such resins are not easily crosslinker by radical crosslinking with peroxides or radiation because cleavage of the main chain due to radicals generally precedes crosslinking, resulting in a failure to obtain a highly expanded foam. A possible countermeasure against this problem is the incorporation of various crosslinking aids comprising a polyfunctional compound, but such an approach is attended by the non-uniformity of crosslinking, the bleeding of low-molecular weight substances, the deterioration of weather resistance, and the deterioration of various physical properties.
In an attempt to overcome the problems associated with the conventional techniques, the inventors previously proposed crosslinkable propylene resins as disclosed in JP-B-64-2139 (the term "JP-B" as used herein means an "examined Japanese patent publication"), JP-A-58-21093 (the term "JP-A" as used herein means an "unexamined Japanese patent publication"), and Japanese Patent Appln. No. Hei-2-134476. The proposed crosslinkable propylene resins have their own utility but are still unsatisfactory as foaming materials. That is, when the resin is kneaded with a chemical foaming agent, the melt viscosity increases causing shearing heat generation, and the resin temperature rises above the decomposition point of the foaming agent. It is likely, as a result, that gas is evolved due to the decomposition of the foaming agent during kneading, thus entrapping gas bubbles of irregular size in the preforms. This being the case, the finally obtained foamed article contains large voids or has a non-uniform cell size, or fails to have a high expansion-rate. Where a physical foaming process is adopted, similar problems are sometimes encountered probably due to the nonuniformity in melt viscosity.
JP-A-58-210930 discloses a process for producing a foam from a composition comprising an .alpha.-olefin-1,4-diene random copolymer and a thermoplastic resin exemplified by polypropylene. The process is characterized as a technique for producing a crosslinked foam from a radical-crosslinkable propylene resin. However, the crosslinking efficiency is not always high, and radical crosslinking is unavoidably accompanied by cleavage of the polymer main chain. Moreover, due consideration is not given to the difference in melting points of the .alpha.-olefin-1,4-diene random copolymer and polypropylene to be blended. On account of these problems, the closed cell size of the resulting foam is not always uniform, and the physical properties of the foam, such as tensile characteristics, still need further improvements.
Further, the process for random copolymerizing propylene, ethylene, and a 1,4-diene has been proposed in JP-A-62-115007 and JP-A-62-115008 (these applications correspond to U.S. Pat. No. 4,680,318), but this technique similarly requires improvements in the crosslinking efficiency and the physical properties of the resulting foam.
In addition, U.S. Pat. No. 4,987,193 discloses a process for producing unsaturated copolymers, including copolymers of propylene and 6-methyl-1,6-octadiene and/or 7-methyl-1,6-octadiene. This reference also contains a general disclosure of compositions which contain such copolymers and a wide range of other polymers, including thermoplastic resins, natural rubbers, and synthetic rubbers. However, this reference contains no suggestion of the criticality of the relationship between the relative properties of the components in a two component foaming composition and the properties of the resulting foam. Thus, this reference does not contain any suggestion of the effect of obtaining foams having improved foam rates and having more uniform cell sizes by controlling the relationship between the melt flow rates and melting points of the polymers in a foaming composition.
Thus, there remains a need for a process for producing plastic foams and the foams produced thereby which are free of the above-described drawbacks.